I've flagged plenty of comments that I agreed with on HN because they were dull and hackneyed.
Frivolous flagging - as you are doing - could eventually get your account privileges removed.
There's a subjective element.
As an example of something I would flag: a one sentence 'hamas supporter!' or 'genocide denier!' accusation in reply to someone's thoughtful comment. If the same sentiment were expressed in a more original way, I might upvote.
Edit: In regard to news stories, sometimes a story breaks and the main and 'new' pages wind up a dozen links to it. At some point, I might flag that. I'm not sure if that's kosher, but there's little purpose in having users wade through identical articles. Maybe @tomhow or @dang can set me straight if they happen to read this.
Unoriginal to who? What's unoriginal to you might be original to someone else. So your justification for flagging only reinforces the groupthink argument even if you don't realize it.
Better to have groupthink that is hostile to groupthink than to have memes.
Some people would like that version of HN more, others less. I probably would close my account.
There might not be a version of this site that would please everybody.